The present invention is directed to the manufacture of ceramic vessels, such as arc discharge vessels for metal halide lamps. As illustrated in FIG. 1, a ceramic vessel 10 for a metal halide lamp has a hollow middle part 12 and may have two hollow capillaries 14 that extend from the middle part. In an arc discharge vessel, the hollow middle part 12 is the arc discharge chamber and the capillaries 14 sealably receive electrodes that extend into the arc discharge chamber. In lamps, the vessel 10 is made of a ceramic material that is transparent or translucent, such as aluminum oxide or aluminum oxynitride.
A common method of molding such vessels has been to form the vessel from several separate parts. For example, a cylindrical central portion may be connected to two capillary end portions by the assembly of three to five individual pieces of ceramic in a process that requires multiple firings.
In lamps, vessel shape is a factor in lamp performance. One improvement is a “bulgy” design that has an elliptical shape, such as two hemispheres separated by a short cylindrical portion. The common method described above is not suitable to make this vessel shape. Bulgy vessels have been made with a two-piece injection molding process in which two half parts are separately molded and then welded together, e.g., as in U.S. Pat. No. 6,620,272 to Zaslavsky et al. While vessels produced with this method are generally acceptable, they have a visible center seam 5 as shown in FIG. 1.
Another known method of molding a one-piece vessel is with a gel casting process that uses a core of a thermoplastic material that is removed by melting. In this method, a fluid suspension is added to the space between an exterior mold and the molded core. The fluid suspension includes a ceramic powder suspended in a liquid medium containing a cross-linking chemical. An activator or catalyst is added to the fluid suspension just prior to filling the mold to initiate the cross-linking process, and the suspension solidifies at completion of the cross-linking. The formed vessel is removed from the exterior mold and then heated to melt the core that drains out of the vessel through an opening in the vessel wall.
A problem with this method has been the residence time needed for the cross-linking to form the vessel, which can vary from several minutes to hours depending on the materials selected. Longer residence times require more molds and more space for high volume production.